The present invention relates to a room temperature-curable silicone rubber composition or, more particularly, to a room temperature-curable silicone rubber composition with a high degree of heat-resistance comprising carbon black as the filler substituting for the siliceous filler used in the art of conventional silicones.
Conventionally, various types of room temperature-curable silicone rubber compositions are known although the heat-resistance thereof is insufficient without exception. A prior art method of improving the heat-resistance of such a silicone rubber composition has been proposed by way of admixing iron(III) oxide. Furthermore, several alternative methods of the above have been proposed, when coloration of silicone compositions due to iron(III) oxide is undesirable, including a method of using ceric oxide or other rare earth metal oxides as the heat-resistance improver (Japanese Patent Publication No. 36-6189), a method of combined use of certain kinds of metal oxides or metal hydroxides therewith (Japanese Patent Publication No. 37-837) and other methods. However, none of these methods can give sufficient heat-resistance at a high temperature such as 280.degree. C. or higher showing a disadvantage of deterioration of the cured silicone composition, for example, after a few days at 300.degree. C. as evidenced by the conversion of a rubbery elastomer into a hard resinous mass.
The inventors of the present invention have unexpectedly arrived at a recognition that the poor heat-resistance of silicone rubber compositions is caused by the siliceous fillers conventionally used as an inorganic filler in silicone rubber compositions after extensive studies with an object to obtain a room temperature-curable silicone rubber composition having excellent heat-resistance. The inventors have completed the present invention on the base of an unexpected discovery that the heat-resistance of silicone rubber compositions can be improved by use of carbon black as an inorganic filler asubstituting for siliceous fillers in combination with iron oxide known as a heat-resistance improver with an object of further upgrading. For example, the inventors have confirmed a fact that the cured silicone rubber poducts obtained from the composition formulated in accordance with the above discovery show little deterioration in the physical properties even after heating in air for 30 days or longer at 300.degree. C.